The present invention relates to a heating apparatus for thermally bonding plastic foils and to a method for operating this apparatus.
In the packaging industry it is known to form band-shaped thermoplastic packaging material into the shape of a cylinder, and to then thermally bond the longitudinal edges of the material to produce a welded seam. The thermal bonding is carried out by placing foils of thermoplastic material to be bonded between a pair of sealing jaws, heating the material to its melting temperature and then pressing the edges of the material together.
An apparatus for heating and processing a sealing jaw against a countersupport is disclosed in U.S. Pat. No. 4,160,689 granted to Willy Altermatt on July 10th, 1979. It is known that such devices operate well if the sealing jaws are relatively short, and it has been proposed to subdivide the sealing jaws into several sections which are aligned with one another and resiliently mounted with respect to a support. However, it has been found that, due to the constant heat and the flow of electric current, the electrical terminals leading to the heating elements tend to oxidize thereby increasing their transfer resistance. This causes a reduction in the heat output of the heating element, and sooner or later the quality of the welded seams suffers. Until this is noted, however, and such flaws can be eliminated, portions of the seams may be welded insufficiently, a risk that is not acceptable with gastight packages.
It is therefore an object of the present invention to provide an apparatus and method for heating the sealing jaws to the required temperature over long periods of time and to maintain operational reliability over longer periods of time despite increasing oxidation of the terminals.